DESCRIPTION

recvfrom() and recvmsg() are used to receive messages from a socket, s,
and may be used to receive data on a socket whether or not it is
connection-oriented.
If from is non-null and the socket is not connection-oriented, the source
address of the message is filled in. fromlen is a value-result parameter,
initialized to the size of the buffer associated with from, and modified
on return to indicate the actual size of the address stored there.
The recv() call is normally used only on a connected socket (see
connect(2)) and is identical to recvfrom() with a null from parameter. As
it is redundant, it may not be supported in future releases.
On successful completion, all three routines return the number of message
bytes read. If a message is too long to fit in the supplied buffer, ex-
cess bytes may be discarded depending on the type of socket the message
is received from (see socket(2)).
If no messages are available at the socket, the receive call waits for a
message to arrive, unless the socket is nonblocking (see fcntl(2)) in
which case the value -1 is returned and the external variable errno set
to EAGAIN. The receive calls normally return any data available, up to
the requested amount, rather than waiting for receipt of the full amount
requested; this behavior is affected by the socket-level options
SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
The select(2) or poll(2) system calls may be used to determine when more
data arrive.
The flags argument to a recv call is formed by ORing one or more of the
values:
MSG_OOB process out-of-band data
MSG_PEEK peek at incoming message
MSG_WAITALL wait for full request or error
MSG_DONTWAIT don't block
The MSG_OOB flag requests receipt of out-of-band data that would not be
received in the normal data stream. Some protocols place expedited data
at the head of the normal data queue, and thus this flag cannot be used
with such protocols. The MSG_PEEK flag causes the receive operation to
return data from the beginning of the receive queue without removing that
data from the queue. Thus, a subsequent receive call will return the same
data. The MSG_WAITALL flag requests that the operation block until the
full request is satisfied. However, the call may still return less data
than requested if a signal is caught, an error or disconnect occurs, or
the next data to be received is of a different type than that returned.
The MSG_DONTWAIT flag requests the call to return when it would block
otherwise. If no data is available, errno is set to EAGAIN. This flag is
not available in strict ANSI or C99 compilation mode.
The recvmsg() call uses a msghdr structure to minimize the number of
directly supplied parameters. This structure has the following form, as
defined in <sys/socket.h>:
struct msghdr {
void *msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
unsigned int msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data, see below */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here msg_name and msg_namelen specify the source address if the socket is
unconnected; msg_name may be given as a null pointer if no names are
desired or required. msg_iov and msg_iovlen describe scatter gather loca-
tions, as discussed in read(2). msg_control, which has length
msg_controllen, points to a buffer for other protocol control related
messages or other miscellaneous ancillary data. The messages are of the
form:
struct cmsghdr {
socklen_t cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by u_char cmsg_data[]; */
};
See CMSG_DATA(3) for how these messages are constructed and decomposed.
As an example, one could use this to learn of changes in the data-stream
in XNS/SPP.
Open file descriptors are now passed as ancillary data for AF_UNIX domain
and socketpair(2) sockets, with cmsg_level set to SOL_SOCKET and
cmsg_type set to SCM_RIGHTS.
The msg_flags field is set on return according to the message received.
It will contain zero or more of the following values:
MSG_OOB Returned to indicate that expedited or out-of-band data
was received.
MSG_EOR Indicates end-of-record; the data returned completed a
record (generally used with sockets of type
SOCK_SEQPACKET).
MSG_TRUNC Indicates that the trailing portion of a datagram was
discarded because the datagram was larger than the
buffer supplied.
MSG_CTRUNC Indicates that some control data were discarded due to
lack of space in the buffer for ancillary data.
MSG_BCAST Indicates that the packet was received as broadcast.
MSG_MCAST Indicates that the packet was received as multicast.

RETURN VALUES

These calls return the number of bytes received, or -1 if an error oc-
curred.

ERRORS

recv(), recvfrom(), and recvmsg() fail if:
[EBADF] The argument s is an invalid descriptor.
[ENOTCONN] The socket is associated with a connection-oriented pro-
tocol and has not been connected (see connect(2) and
accept(2)).
[ENOTSOCK] The argument s does not refer to a socket.
[EAGAIN] The socket is marked non-blocking, and the receive opera-
tion would block, or a receive timeout had been set, and
the timeout expired before data were received.
[EINTR] The receive was interrupted by delivery of a signal be-
fore any data were available.
[EFAULT] The receive buffer pointer(s) point outside the process's
address space.
[EHOSTUNREACH] A socket operation was attempted to an unreachable host.
[EHOSTDOWN] A socket operation failed because the destination host
was down.
[ENETDOWN] A socket operation encountered a dead network.
In addition, recv() and recvfrom() may return the following error:
[EINVAL] len was larger than SSIZE_MAX.
Also, recv() may return the following error:
[ECONNREFUSED] The socket is associated with a connection-oriented pro-
tocol and the connection was forcefully rejected (see
connect(2)).
And recvmsg() may return one of the following errors:
[EINVAL] The sum of the iov_len values in the msg_iov array over-
flowed an ssize_t.
[EMSGSIZE] The msg_iovlen member of msg was less than 0 or larger than
IOV_MAX.